First the really good news - I'm incredibly lucky with the weather. I once had a "streak" of 149 weddings in a row where we were able to go ahead in the planned location without having to move indoors because of rain.
But if it does rain and you're getting married at a venue - relax! Even though an outdoor wedding is fabulous, venues always have a Plan B location in case the weather is bad. There's always a spare room, a covered courtyard, a wide verandah or similar and the staff at your venue will be experts in transforming this spot into a gorgeous ceremony area in double quick time.
If you're not getting married at a venue, make sure you have an alternative location organised in case it rains. Beach weddings are beautiful, but noone wants to spend their big day cold, damp and miserable in the company of grumpy, wet guests. If you have to change location, make sure you notify all your guests early enough for them to arrive at the new location in plenty of time.
It's really important to notify your celebrant as soon as you can, because if you change location, your marriage certificates have to be reprinted with the new address for the ceremony. Remember, your celebrant needs time to change the paperwork, drive to your alternative location and be set up and ready before your guests start to arrive.